Overvoltage protection devices have a protective element which generally includes a non-linear element (varistor) which, due to its loading of electric current and by an impulse loading of a protected network, gradually decreases the value of its resistance. Due to this, the current running through the protective element increases, and its temperature increases as well. Therefore, the overvoltage protection includes a temperature cut-off device which serves to disable the protective element due to its temperature, preventing the protective element from properly fulfilling its function. Disabling the protective element from the network is indicated either visually directly on the overvoltage protection or remotely by transmission of a suitable signal. Once the protective element is cut off from the network, the network is no longer protected, so it is necessary to regain the protected status by replacing the protective element of overvoltage protection.
The visual indication of the status of overvoltage protection is required, especially for overvoltage protection of category II equipment according to the IEC 61643-11. This status indicator distinguishes between two modes of status, the “good one”—green color, and the “fault one”—red color. The status modes may be expressed even differently than through this colorful resolution. The disadvantage of such status indicators is that it does not identify when the overvoltage protection is already partially degraded but not yet disabled from the protected circuit by means of a built in cut-off device. Due to the fact that only the enabled or disabled status of the protected circuit is indicated, a situation may occur when the overvoltage protection is degraded due to deterioration t or disabled before the non-functioning or disabled overvoltage protection is replaced by a functioning one, causing the respective electrical circuit to be not protected, and thus increasing the hazard of damage of the non-protected electrical equipment due to an overvoltage condition.
There is a known solution in which between the phase and neutral or ground wire there are included two parallel connected varistors, with each varistor having its own cut-off device from the protected circuit. The first varistor is cut off due to melting of the temperature fuse which causes the pressure spring to move the shifting part to act upon the swiveling part to block about half of the overvoltage protection signal which provides optical information that the overvoltage protection device is partially deteriorated. The shifting part, changes its position to simultaneously activate the remote status indication of overvoltage protection. When the second varistor is cut off, the entire overvoltage protection signal is blocked through the same mechanism to create the visual indication that the entire overvoltage protection for the protected circuit is disabled.
Considerable complexity and coupling of several functional elements results in higher production costs which is disadvantageous for this solution.
There is another known solution which signals partial deterioration of overvoltage protection by means of a pair of parallel connected varistors equipped with cut-off mechanisms, each having its own spring. The function of both cut-off mechanisms always depends on the temperature of both varistors. One of the cut-off mechanisms disconnects at a lower temperature of the varistors than the second one. The status indicator shows a green light in case the overvoltage protection is in flawless status. As a result of the operation load and aging of the varistors, the varistors warm up until the cut-off device with the lower temperature setting actuates to screen the status indicator and produce a yellow color indication, creating a visual indication of partial deterioration of overvoltage protection which is, henceforth functioning. Simultaneously through movement of the cut-off mechanism, the remote status indication of overvoltage protection is activated. As a result of further increasing of varistor temperature, upon co-acting of the second spring, the second cut-off mechanism actuates to screen the status indicator and produce a red color to indicate that the overvoltage protection is totally deteriorated and disabled from the protected circuit.
Disadvantage of this solution is its considerable complexity of a pair of independent complete cut-off mechanisms which results in high costs for such overvoltage protection.
The objective of the invention is to eliminate or at least to minimize the disadvantages of the background art.